Field of the Invention
This invention relates to a method for forming a pattern using a photomask in a fine processing process, and a pattern-forming apparatus.
Recently, as a semiconductor memory has become larger in its capacity and a CPU processor has become faster and more integrated, finer photolithographic techniques have been essentially required. In general, the lower limit of fine processing in a photolithography apparatus is about the wavelength of the light used. The wavelength of light used in a photolithography apparatus has, therefore, become shorter. Now, a near ultraviolet laser is used, allowing us to conduct fine processing to about 0.1 μm.
Although photolithography has become finer, there are many problems to be solved such as a further shorter wavelength of the laser and development of a lens for such a shorter wavelength band, for fine processing of 0.1 μm or less.
On the other hand, there has been proposed an apparatus for fine processing utilizing a configuration of a nearfield optical microscope (hereinafter, referred to as an “SNOM”) for achieving fine processing of 0.1 μm or less with light. It is a technique in which a photoresist is subjected to a local exposure over a light wavelength limit using evanescent light leaking out from a fine aperture of 0.1 μm or less.
However, in any lithography apparatus with an SNOM configuration, fine processing is conducted like pen writing, using a single processing probe or several processing probes, so that throughput may not be improved much.
To solve the above-described problem, there has been suggested a technique that a photomask having a pattern in which evanescent light leaks out between shielding films is tightly placed on a photoresist on a substrate and is subjected to exposure, whereby a fine pattern on the photomask is transferred to the photoresist at one time (Japanese Patent Application Laid-Open No. 11-145051).
A pattern required in actual lithography is a combination of patterns with various sizes. For example, it is often a combination of patterns larger than an exposure wavelength with small patterns formed by evanescent-light exposure.
Thus, when attempting exposure using a mask where apertures with sizes for both evanescent-light exposure and propagating-light exposure are provided on a single matrix, a sensitivity level of the photoresist is dispersed depending on a pattern, so that it is difficult to form a uniform pattern because the light intensity of the evanescent light is much weaker than that of a the propagating light.
The phenomenon will be detailed with reference to FIGS. 9A to 9C. FIG. 9A shows a mask in which an aperture 213 is disposed near an aperture 214. In this case, the photoresist is a positive type, and when a negative type photoresist is used, the same result is obtained.
The aperture 213 through which the evanescent light is transmitted as a main exposure component gives a considerably smaller quantity of transmitted light than that of incident light, depending on its width, shape and its spatial relationship with other patterns.
When the above-described mask is used for exposure while controlling a quantity of the incident light to the mask such that the photoresist reacts by a quantity of the evanescent light, the quantity of light from the aperture 214 is so excessive that a photoresist pattern 217 by exposure from the aperture may become larger than the mask pattern and may cover a portion of a photoresist pattern by the exposure from the minute aperture as shown in FIG. 9B.
On the other hand, the above-described mask is used for exposure while controlling a quantity of the incident light to the mask such that the photoresist reacts by a quantity of the propagating light from the aperture 214, and the quantity of the evanescent light from the aperture 213 is so small that the photoresist may inadequately react, whereby a photoresist reaction portion by the evanescent light may not be formed, as shown in FIG. 9C.
Similarly, when a photoresist is made thicker, its reaction state may become uneven in a photoresist reaction portion 206 by the evanescent light and in a photoresist reaction portion 207 by the propagating light, along the depth direction of a photoresist 203 and along the plane direction of a substrate 204, as shown in FIG. 3A. As a result, only the photoresist reaction portion 207 by the propagating light is formed by development after exposure, as shown in FIG. 3B. Furthermore, the photoresist reaction portion 207 by the propagating light may expand on the substrate 204.
In such a case, patterns may be formed by separate processes; a fine pattern is formed by exposure to evanescent light and then a larger pattern is formed by exposure to usual propagating light. However, it may lead to a higher cost and a lower throughput due to an increase in the numbers of masks and of process steps.